Dopaminergic function in spinocerebellar ataxia type 6 patients with and without parkinsonism

BACKGROUND

Although pure cerebellar ataxia is usually emphasized as the characteristic clinical feature of spinocerebellar ataxia type 6 (SCA6), parkinsonism has been repeatedly described in patients with genetically confirmed SCA6.

METHODS

We conducted a positron emission tomography study using a combination of [¹⁸F]fluoro-L-dopa for dopamine synthesis and [¹¹C]raclopride for dopamine D2 receptor function on six genetically confirmed SCA6 patients, both with and without parkinsonism. To the best of our knowledge, this is the first dopamine receptor imaging study of patients with SCA6.

RESULTS

Most patients had somewhat decreased dopaminergic function, and this decrease was significant in the caudate nucleus. In addition, one SCA6 patient with parkinsonism had whole striatal dysfunction of both dopamine synthesis and dopamine D2 receptor function.

CONCLUSIONS

The pathology of SCA6 may not be restricted to the cerebellum, but may also be distributed across various regions, including in both presynaptic and postsynaptic dopaminergic neurons to some degree. Patients with SCA6 may show apparent parkinsonism after the progression of neurodegeneration.

Polyglutamine length-dependent toxicity from α1ACT in Drosophila models of spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disease that results from abnormal expansion of a polyglutamine (polyQ) repeat. SCA6 is caused by CAG triplet repeat expansion in the gene CACNA1A, resulting in a polyQ tract of 19-33 in patients. CACNA1A, a bicistronic gene, encodes the α1A calcium channel subunit and the transcription factor, α1ACT. PolyQ expansion in α1ACT causes degeneration in mice. We recently described the first Drosophila models of SCA6 that express α1ACT with a normal (11Q) or hyper-expanded (70Q) polyQ. Here, we report additional α1ACT transgenic flies, which express full-length α1ACT with a 33Q repeat. We show that α1ACT33Q is toxic in Drosophila, but less so than the 70Q version. When expressed everywhere, α1ACT33Q-expressing adults die earlier than flies expressing the normal allele. α1ACT33Q causes retinal degeneration and leads to aggregated species in an age-dependent manner, but at a slower pace than the 70Q counterpart. According to western blots, α1ACT33Q localizes less readily in the nucleus than α1ACT70Q, providing clues into the importance of polyQ tract length on α1ACT localization and its site of toxicity. We expect that these new lines will be highly valuable for future work on SCA6.